This invention relates to the conversion of coal and similar carbonaceous solids in the presence of an alkali metal-containing catalyst and is particuarly concerned with the recovery of alkali metal constituents from spent solids produced during coal gasification and similar operations and their reuse as constituents of the alkali metal-containing catalyst.
It has long been recognized that certain alkali metal compounds can be employed to catalyze the gasification of carbonaceous material such as coal and other carbonaceous solids. Studies have shown that potassium carbonate, sodium carbonate, cesium carbonate and lithium carbonate will substantially accelerate the rate at which steam, hydrogen, carbon dioxide, oxygen and the like react with bituminous coal, subbituminous coal, lignite, petroleum coke, organic waste materials and similar carbonaceous solids to form methane, carbon monoxide, hydrogen, carbon dioxide and other gaseous products. Other alkali metal salts such as alkali metal chlorides, however, have a low catalytic activity when compared to that of the corresponding carbonate and will only accelerate the gasification reactions at a fraction of the rate obtainable with alkali metal carbonates. It has been found that of the alkali metal carbonates, cesium carbonate is the most effective gasification catalyst, followed by potassium carbonate, sodium carbonate and lithium carbonate in that order. Because of the relatively high cost of cesium carbonate and the low effectiveness of lithium carbonate, most of the experimental work in this area which has been carried out in the past has been directed toward the use of potassium and sodium carbonate. The catalytic activity of sodium carbonate, however, is substantially lower than that of potassium carbonate, therefore attention has been focused in the past on the use of potassium carbonate as a gasification catalyst.
Coal gasification processes and similar operations carried out in the presence of alkali metal compounds at high temperatures generally result in the formation of chars and alkali metal residues. Coal and other carbonaceous solids used in such operations normally contain mineral constituents that are converted to ash during the gasification process. Although the composition of ash varies, the principle constituents, expressed as oxides, are generally silica, alumina and ferric oxide. The alumina is usually present in the ash in the form of aluminosilicates. Studies indicate that at least a portion of the alkali metal compounds that are used as gasification catalyst constituents react with the aluminosilicates and other ash constituents to form alkali metal residues containing water-soluble alkali metal compounds such as carbonates, sulfates and the like and water-insoluble, catalytically inactive materials such as alkali metal aluminosilicates. Thus the chars produced during coal gasification and similar conversion processes will contain in addition to carbonaceous material and ash, alkali metal residues comprised of both water-soluble alkali metal constituents and water-insoluble alkali metal constituents. It is generally advisable to withdraw a portion of the char from the reaction zone during gasification and similar operations in order to eliminate the ash and alkali metal residues and prevent them from building up within the reaction zone or other vessels in the system. Elutriation methods and other techniques for separating char particles of relatively high ash content and returning particles of relatively low ash content to the reaction zone in order to improve the utilization of carbon in such processes has been suggested.
In gasification and other processes referred to above that utilize alkali metal-containing catalysts, the cost of the alkali metal constituents is a significant factor in determining the overall cost of the process. In order to maintain catalyst cost at a reasonable level, it is essential that the alkali metal consituents be recovered and reused. Since the alkali metal is present in the form of both water-soluble and water-insoluble compounds, not all of the alkali metal constituents can be recovered by water washing alone. It has been proposed to recover the water-insoluble alkali metal constituents by treating the char particles with a solution of sulfuric acid, hydrochloric acid, or formic acid. Although hydrochloric and sulfuric acid are effective in extracting the alkali metal from the char particles, such extraction results in the formation of alkali metal chlorides and alkali metal sulfates, both of which have now been found to be poor gasification catalysts. Extraction with formic acid, on the other hand, results in the formation of an alkali metal formate which is an active gasification catalyst. Formic acid, however, is only effective in extracting a small portion of the water-insoluble alkali metal constituents from the char.